1. Field of the Invention
The present invention relates to a multilayer printed wiring board which has high connection reliability of interlayer connection vias and permits high-density wiring and a method of manufacturing this multilayer printed wiring board.
2. Description of the Related Art
Methods of forming a circuit in a printed wiring board are divided into the subtractive method which involves forming an etching resist on a metal conductor layer of copper foil etc. and forming a wiring circuit by etching the part of metal conductor layer not covered by this etching resist; and the additive method which involves forming a plating resist of a pattern reverse to a circuit and forming a wiring circuit by precipitating a plating on an opening of this plating resist.
Because the manufacturing steps in the subtractive method are easier than in the additive method, it is possible to manufacture circuits at low cost. However, in the subtractive method it is necessary to apply a nonelectrolytic plating and an electrolytic plating to the whole insulating substrate in forming full through holes and blind via holes and the like (hereinafter referred to as via holes) and, therefore, the thickness of the conductor layer to be etched equals to the sum of metal foil plus a plating. Thus, the subtractive method was not very suitable for the formation of a good fine wiring circuit, for example, a wiring circuit having L/S (line/space) of not more than 75 μm/75 μm.
In contrast, the additive method, which is favorable for the formation of a fine wiring circuit, requires that a wiring circuit be formed by precipitating a plating on an insulating layer and, therefore, compared to a case as in the subtractive method where an insulating substrate in which metal foil has been laminated on an insulating layer is processed, the additive method had problems such that the adhesion of the wiring circuit is poor.
Furthermore, in terms of the design of printed wiring circuits, wiring circuits are formed nonuniformly in the plane of the substrate and, therefore, when wiring circuits are formed by selective plating as in the additive method, currents are concentrated more than necessary in portions of coarse wiring circuits and variations occur in the thickness of wiring circuit, thereby posing problems such that it is difficult to ensure impedance matching.
As methods of manufacturing printed wiring boards to solve such problems, there has already been reported, for example, a technique as disclosed in JP-A-2004-319994.
That is, the method including forming a wiring circuit in the part of metal foil beforehand, forming then at least a barrier layer substantially on the whole upper area followed by forming a nonthrough hole, and applying first a nonelectrolytic plating and then an electrolytic plating substantially to the whole area including via holes, thereby forming via holes. By this method it is possible to form via holes of high connection reliability at the same time with the formation of a fine wiring circuit.
In the above-described conventional method, in particular, materials with a metal foil thickness of 12 μm, 9 μm and 7 μm are used when the line/space of a fine wiring circuit decreases from 30 μm to 20 μm and 10 μm, respectively.
In this method, when a wiring circuit is refined and the thickness of metal foil is reduced, an insulating layer is laminated on an upper layer and the wiring circuit and via holes are connected. When a wiring circuit is used as a via hole bottom pad of an insulating layer of the upper layer, because of thinness of the metal foil, this posed the problem that when a nonthrough hole is formed by laser processing for example, it becomes impossible to form the nonthrough hole which pierces through the via hole bottom pad. That is, a depression as indicated by C in FIG. 4 is formed, making it impossible to maintain interlayer connection.
The metal foil height of each layer in a case where a via hole is superposed on a via hole differs from that in a case where a via hole is formed on metal foil. For example, when a via hole is provided on metal foil, this posed the problem that the interior of the via hole is not sufficiently filled with a metal plating, resulting in the formation of a recessed portion. That is, a recess as indicated by D in FIG. 5 is formed.
In view of the above-described problems and actual situation, the present invention has as its object the provision of a multilayer printed wiring board provided with flat via holes.